The examination of pollutants in the exhaust streams of combustion engines is increasingly important area of interest. One key pollutant is soot created during the combustion cycle. There are numerous techniques for measuring the soot, some of which are laser based like the well known laser induced incandescence, LII. However, the equipment used for LII and similar techniques are difficult to calibrate due to the lack of a reliable calibration standard. This invention seeks to provide an improved calibration standard.